The value 'stable' is invalid for APT::Default-Release as such a release is not available in the sources
Can't call method "policy" on an undefined value at /usr/bin/apt-show-versions line 56.
E: Problem executing scripts APT::Update::Post-Invoke-Success 'test -x /usr/bin/apt-show-versions || exit 0 ; apt-show-versions -i'
E: Sub-process returned an error code